1) This from its own website: (https://www.bydglobal.com/en/NewEnergy.html):

"BYD is the world's leading producer of rechargeable batteries: NiMH batteries, Lithium-ion batteries and NCM batteries. BYD owns the complete supply chain layout from mineral battery cells to battery packs."

This would help contribute to lower prices.

https://www.abiresearch.com/blogs/2024/01/26/how-byd-passed-tesla-as-ev-market-leader/

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2) The Chinese Government heavily subsidises BYD and other Chinese EV companies.

https://electrek.co/2024/04/12/china-gave-byd-an-incredible-3-7-billion-to-win-the-ev-race/

https://asia.nikkei.com/Spotlight/Electric-cars-in-China/China-gives-EV-sector-billions-of-yuan-in-subsidies

https://www.euronews.com/my-europe/2024/06/13/what-brussels-found-when-it-probed-chinese-subsidies-and-electric-cars

https://www.sciencedirect.com/science/article/pii/S1809203916300018

3) Cheaper labour costs than most of the legacy car manufacturing hubs (Japan, Germany, Italy, other parts of Europe, and, in recent years, South Korea).

Yes, China's wages has also increased since the turn of the century, but are still competitive.

There are other factors that play that I won't mention for brevity...amongst other reasons.